[ffmpeg] auto detect outgoing_time_base for Encoder

[NewYearPrism]

Tell us about your Wish

For example, matroska (mkv) uses a fixed tbn of 1k (1/1000 time_base). if output is matroska, outgoing_time_base must be set to 1/1000 or video will play at a wrong speed.
It will be more convenient if we don't need to explicitly specify the time base when not necessary.

Is your feature related to a bug

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Additional Info

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Participation

• I am willing to submit a pull request to implement this